The known plants for the production of extruded tubular films generally comprise an extrusion head, from which the molten plastic material exits through an annular gap, thus forming a tube which is then subjected to a stretching action to achieve the material's orientation in two directions.
The longitudinal orientation of the material is achieved by traction rollers which advance the material at a speed superior to that at which ii exits the drawplate, while the transversal orientation is attained by inflating the tube.
This orienting operation endows the material with the thermal retracting capability needed to control it with precision. A known process in this regard is that of exposing an already oriented tube to a further stabilizing process which includes reheating and inflating the tube to a pre-established diameter and subsequently cooling it in a cooling chamber containing a calibrating device capable of determining the finished diameter of the oriented and stabilized tube.
The individual devices used for these operations are in themselves known, and a detailed description is therefore unnecessary.
A typical plant for the production of bidirectionally oriented tube generally includes the following equipment:
one or more extruders injecting the molten materials in an extruding device; PA0 an extrusion head in which one or several layers of material is conveyed under controlled pressure and temperature conditions to an interchangeable drawplate from which it exits through a calibrated annular gap to pass into a cooling section; PA0 an orienting section comprising a pair of inlet traction rollers, an oven for heating the tube, devices capable of inducing the expansion of the tube inside the oven, a cooling chamber and outlet traction rollers for a longitudinal stretching of the tube.
All these equipment items are controlled by electronic systems, also of a known type, which act on various parameters such as temperature, pressure, extrusion speed and tube traction speed, so as to achieve the desired result. At the present state of the art, the tube thus produced is subjected to a stabilization stage in a separate plant, which also comprises a pair of tube feeding rollers, a tube heating oven, means capable of inducing an inflation of the tube and a cooling section equipped with a calibrator which determines the diameter of the finished tube.
At present, therefore, the tube is produced in two stages, which provide means for an extrusion and stretching of the tube and its subsequent stabilization, respectively. This involves a waste of space and a lenghtening of the processing times, because the produced and oriented tube must be collected in spools, inserted into the stabilizing plant, and collected as a finished product at the outlet.
In the known plants the transversal stretching process, both in the extruding and in the stabilizing stage, is carried out by pumping up the tube with a predetermined volume of air, so as to induce its expansion to the desired degree.
This air therefore determines a sort of bubble limited at its extremities by the tube's pair of feeding and traction rollers, producing a configuration in which the tube in fact flows around this air bubble trapped in its interior between the two pairs of rollers.
This solution, while effective, is still not free of shortcomings, as it is practically impossible to prevent some of the air from escaping while the tube is passing through the rollers.
As a consequence of all this, the pressure inducing the expansion of the tube keeps dropping, albeit slowly, while the processing proceeds. This makes it necessary to periodically stop the plant and reintroduce an adequate amount of air into the tube to reestablish the conditions envisioned to induce the required expansion.